1. Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method thereof, and more particularly to the structure of a liquid crystal display panel (liquid crystal display element) suitable for removing undesired charge remaining on electrodes which apply an electric field to a liquid crystal layer thereof, for promptly changing over images on a display screen and for promptly erasing images at the time of completion of display operation and a method for manufacturing thereof.
2. Description of the Related Art
Liquid crystal display devices have been popularly used as display devices for personal computers, monitors, television sets and the like. The liquid crystal display device includes a liquid crystal display panel which is comprised of a pair of substrates, a liquid crystal layer sandwiched between a pair of substrates (a layer made of liquid crystal composition sealed between a pair of substrates), and a group of electrodes which are formed on a main surface of at least one of these pair of substrates which faces the liquid crystal layer in an opposed manner. The display operation of the liquid crystal display device is performed such that an electric field applied to (the inside of the liquid crystal layer by a group of electrodes is controlled in response to information to be displayed so as to modulate the light transmissivity of the liquid crystal layer. In the above-mentioned main surface of the substrate of the liquid crystal display panel, a region where the light transmissivity of the liquid crystal layer is modulated (a region where the display operation is performed) is referred to as a display screen, a display region or an effective display region.
The liquid crystal display device is classified into two types, that is, the active matrix type and the passive matrix type depending on the behavior of liquid crystal molecules in the inside of the above-mentioned liquid crystal layer during the display operation and the electrode structure in the inside of the above-mentioned liquid crystal display panel adapted to the behavior. The former liquid crystal display device is characterized in that an active element (switching element) is formed on each pixel which constitutes the display region. As such an active element, for example, a thin film transistor (TFT) or a thin film diode (TFD) is used.
An example of the active matrix type liquid crystal display device is explained in conjunction with an equivalent circuit diagram of a liquid crystal display device using thin film transistors shown in FIG. 15.
As shown in FIG. 15, on a display screen 50 (a region surrounded by a broken line) of the liquid crystal display device, a plurality of scanning signal lines 10 which extend in the x direction and are arranged in parallel in the y direction which intersects the x direction and a plurality of video signal lines (also referred to as “data lines”) 12 which extend in the y direction and are arranged in parallel in the x direction are formed. Further, on the display screen 50, a plurality of thin film transistors TFT each of which is connected to one of a plurality of scanning signal lines 10 and one of video signal lines 12 are formed. A plurality of thin film transistors TFT are formed of so-called field effect type transistors which are switched in response to a voltage applied to gate electrodes, wherein one of a plurality of scanning signal lines 10 is connected to each gate electrode. One of a plurality of video signal lines 12 is connected to a drain electrode of each thin film transistor TFT, while a pixel electrode which applies an electric field to the liquid crystal layer is connected to a source electrode of each thin film transistor TFT. The pixel electrode is indicated as capacitance CLC by being coupled with a counter electrode (also referred to as a common electrode) which generates an electric field to be applied to the liquid crystal layer along with the pixel electrode. A video signal voltage Y1, Y2, Y3, . . . Yend which is generated in response to an image to be displayed is supplied to a plurality of thin film transistors TFT (arranged in the y direction in FIG. 15) to which the video signal lines 12 are connected through the video signal lines 12, while each thin film transistor TFT supplies the above-mentioned video signal voltage X1R, X1G, X1B, . . . XendB to the pixel electrode in accordance with the timing at which the scanning signal voltage Y1, Y2, Y3, . . . Yend is applied to the gate electrode through one of the scanning signal lines 10. Accordingly, on the display screen 50 of the liquid crystal display device, the pixels PIX each of which includes one of the plurality of thin film transistors TFT and the capacitance CLC to which the video signal voltage is applied through the thin film transistor TFT are formed two-dimensionally. Here, as described above, as viewed from the gate electrode (a channel layer in which the movement of charge is controlled due to the gate electrode) of the thin film transistor TFT, the electrode arranged at the video signal line 12 side is set as the drain electrode and the electrode at the pixel electrode (capacitance CLC) side as the source electrode. However, the naming of these electrodes can be exchanged based on the relative relationship between potentials of both electrodes. In this specification, for the sake of convenience, the electrode of the thin film transistor TFT at the video signal line 12 side is referred to as the drain electrode and the electrode of the thin film transistor TFT at the pixel electrode side is referred to as the source electrode.
On the other hand, to the counter electrode which constitutes the capacitance CLC together with the pixel electrode, the reference voltage Vcom is supplied through a reference voltage line 11. Depending on the mode of applying voltage to the liquid crystal layer (modulation of optical transmissivity of the liquid crystal layer), the counter electrodes and the reference voltage lines 11 are formed on either a substrate (also referred to as a TFT substrate) on which the above-mentioned scanning signal lines 10, the video signal lines 12, the thin film transistors TFT and the pixel electrodes are formed or another substrate which faces the TFT substrate in an opposed manner while sandwiching a liquid crystal layer therebetween. Since the former liquid crystal display device generates an electric field in a liquid crystal layer along a main surface of the TFT substrate, the liquid crystal display device is referred to as an in-planes-switching (abbreviated as IPS) type liquid crystal display device or a lateral electric field type liquid crystal display device. On the other hand, since the latter liquid crystal display device generates an electric field in a liquid crystal layer along the thickness direction, the device is also referred to as a vertical electric field type liquid crystal display device. Here, in the vertical electric field type liquid crystal display device, there may be a case in which one counter electrode corresponds to a plurality of pixel electrodes (for example, all pixel electrodes arranged within the above-mentioned display screen SCR) and the above-mentioned capacitance CLC is formed for every pixel or the counter electrode also performs a function of the reference voltage line 11 on a main surface of another substrate which faces the TFT substrate in an opposed manner. Such a vertical electric field type structure is applicable to a liquid crystal display device using twisted nematic liquid crystal which gradually twists a long axis direction of liquid crystal molecules in the inside of the liquid crystal layer from the TFT substrate to the substrate which faces the TFT substrate in an opposed manner (a so-called TN type liquid crystal display device) and a so-called vertically aligned type (VA type) liquid crystal display device which aligns a long axis of liquid crystal modules with respect to the main surface of the TFT substrate with an inclination of a given angle.
The above-mentioned scanning signal lines 10 are respectively electrically connected to output terminals of a driving circuit (a vertical scanning circuit or also referred to as a gate driver) V-DRV and receive the scanning signals Y1, Y2, Y3, . . . Yend. The above-mentioned video signal lines 12 are respectively connected to output terminals of a driving circuit (a video signal driving circuit or also referred to as a drain driver) H-DRV different from the driving circuit V-DRV and receive the video signals X1R, X1G, X1B, . . . XendB. Data on images to be displayed on the liquid crystal display device is inputted to a control circuit (also referred to as a timing converter) TCON from the outside and the scanning signals and the video signals (possibly including gray scale signals) which are suitable for operation of the liquid crystal display device are generated.
Further, the pixel PIX shown in FIG. 15 is also provided with another capacitance Cad besides the above-mentioned capacitance CLC. The capacitance Cadd is also referred to as an additional capacitance or a storage capacitance and is provided for holding a charge supplied to the pixel electrode of each pixel in response to the video signal until a point of time that a charge corresponding to a next video signal is supplied to the pixel electrode.